Retrofocus type wide angle lens comprising a fixed group and movable corrective and rear groups

ABSTRACT

Described herein is a retrofocus type wide angle lens system composed of a front group with a negative power and a rear group with a positive power, in which the front group is divided into a fixed group starting with a negative meniscus lens having a convex surface on the object side and terminating with a negative lens element having a concave surface on the image-forming side, and a corrective group starting with a positive lens element having a convex surface on the object side. The fixed group which is constituted by larger and heavier lens elements is held stationary relative to an image forming plane, while the corrective group of smaller and light-weight lens elements is movable in the direction of the optical axis concurrently with the rear group to vary the width of the air space between the respective groups at the time of focusing, thereby effectively correcting various aberrations which would otherwise occur in a conspicuous degree in infinite to near distance focusing.

BACKGROUND OF THE INVENTION

1. Field of the Art

This invention relates to a wide angle lens for television cameras, andmore particularly to a retrofocus type wide angle lens which is capableof making corrections to minimize variations in aberrations especiallyin field curvature and astigmatism which take place when focusing to anobject at a close distance.

2. Description of the Prior Art

To meet a demand for a back focal length which is several times longerthan the focal length, television camera lenses generally employ aretrofocus type lens system which is composed of a front group of lenselements with a negative refractive power and a rear group with apositive power. In focusing, it is the general practice with aretrofocus type lens of this sort to shift the position of the wholelens system forward or in a direction away from the image-forming plane.In this connection, there has also been known in the art to correct thefield curvature, which varies in a marked degree especially in neardistance focusing, by shifting the position of the negative front grouprearward or toward the image-forming plane while sifting the whole lenssystem forward.

In the former case where the whole lens system is shifted forward at thetime of focusing, it is necessary to correct aberrations on the basis ofa reference object point located between infinite distance and closedistances, in view of the lens characteristics of the asymmetricarrangement combining a negative front group with a positive rear group.Therefore, aberrations are suitably corrected at the reference objectdistance, but except the object point at the reference distance it isdifficult to obtain images of good quality due to variations inaberrations especially in field curvature, which occur in aconspicuously magnified degree in infinite to near distance focusing.For example, if the whole lens system is shifted forward, variousaberrations occur even in case of an optical system in one of theembodiments of the invention, as seen in the comparative examples shownin FIGS. 5, 9, 13 and 17 which will be discussed hereinlater.

On the other hand, in the latter case where the front group is shiftedbackward while shifting the whole lens system forward, it is possible tocorrect the field curvature which varies to a considerable degree infocusing to an object at the nearest distance. However, since thissystem is a wide angle lens with an extremely short focal length ascompared with the back focal length, the negative refractive power ofthe front group becomes too strong to correct aberrations such as fieldcurvature and astigmatism. That is to say, for correction of theseaberrations, there have to be incorporated a larger number of lenselements of a larger diameter, which will result in equipments of heavyweight and awkward to handle in focusing operations, in addition to aproblem that the correction of aberrations in near distance focusing isstill unsatisfactory.

SUMMARY OF THE INVENTION

The present invention contemplates to eliminate the above-mentionedproblems or drawbacks of the prior art, and has as its object theprovision of a retrofocus type wide angle lens which can correctvariations of aberrations in infinite to near distance focusing.

It is another object of the present invention to provide a retrofocustype wide angle lens which can correct aberrations securely over theentire focusing range from infinite to near distance to permitultra-wide angle shooting of high performance quality.

In accordance with the present invention, the above-stated objectivesare achieved by the provision of a retrofocus type wide angle lenssystem which is composed of a negative front group G₁,2 and a positiverear group G₃, characterized in that: the front group G₁,2 isconstituted by a fixed group G₁ starting with a negative meniscus lenshaving a convex surface on the object side and terminating with anegative lens element having a concave surface on the imaging side, anda corrective group G₂ starting with a positive lens element having aconvex surface on the object side, the fixed group G₁ being fixedrelative to an image-forming plane while the corrective group G₂ and therear group G₃ being movable concurrently in the direction of the opticalaxis to vary the width of the air space between the respective groupsfor focusing purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill become apparent from the following description and the appendedclaims, taken in conjunction with the accompanying drawings which showsome preferred embodiments of the invention and in which:

FIG. 1 is a diagrammatic illustration of a retrofocus type wide anglelens according to the invention, showing shifted lens positions infocusing;

FIG. 2 is a schematic illustration of the lens construction in a firstembodiment of the invention;

FIGS. 3(a) to 3(c) are diagrams of aberrations at the time of infinitedistance focusing in the first embodiment of the invention;

FIGS. 4(a) to 4(c) are diagrams of aberrations at the time of neardistance focusing in the first embodiment of the invention;

FIGS. 5(a) to 5(c) are diagrams of aberrations in near distancefocusing, as observed when the whole lens system of the first embodimentis shifted forward;

FIG. 6 is a schematic illustration of the lens construction in a secondembodiment of the invention;

FIGS. 7(a) to 7(c) are diagrams of aberrations at the time of infinitedistance focusing in the second embodiment of the invention;

FIGS. 8(a) to 8(c) are diagrams of aberrations at the time of neardistance focusing in the second embodiment of the invention;

FIGS. 9(a) to 9(c) are diagrams of aberrations in near distancefocusing, as observed when the whole lens system of the secondembodiment is shifted forward;

FIG. 10 is schematic illustration of the lens construction in a thirdembodiment of the invention;

FIGS. 11(a) to 11(c) are diagrams of aberrations at the time of infinitedistance focusing by the third embodiment of the invention;

FIGS. 12(a) to 12(c) are diagrams of aberrations at the time of neardistance focusing by the third embodiment of the invention;

FIGS. 13(a) to 13(c) are diagrams of aberrations in near distancefocusing, as observed when the whole lens system of the third embodimentis shifted forward;

FIG. 14 is a schematic illustration of the lens construction in a fourthembodiment of the invention;

FIGS. 15(a) to 15(c) are diagrams of aberrations at the time of infinitedistance focusing by the fourth embodiment of the invention;

FIGS. 16(a) to 16(c) are diagrams of aberrations at the time of neardistance focusing by the fourth embodiment of the invention; an

FIGS. 17(a) to 17(c) are diagrams of aberrations in near distancefocusing, as observed when the whole lens system of the fourthembodiment is shifted forward.

DESCRIPTION OF PREFERRED EMBODIMENTS

Hereafter, the invention is described in greater detail by way of thepreferred embodiments shown in the drawings.

FIG. 1 schematically shows the construction of a lens system accordingto the invention along with its shifted mode, in which indicated at G₁,2is a front group with a negative combined focal length, at G₃ is a reargroup with a positive combined focal length, at X is an optical axis andat I is an image-forming plane. In this instance, the front group G₁,2is divided into a first subgroup consisting of lens elements which arelarger in outer diameter and weight and a second subgroup consisting oflens elements which are relatively small in outer diameter and weight.The larger and heavier subgroup on the object side serves as a fixedgroup G₁, and the smaller and light-weight subgroup on the image-formingside serves as a corrective group G₂ which is movable in the directionof the optical axis X.

This lens system brings images of objects to focus basically by shiftingthe position of the rear group G₃ forward or rearward along the opticalaxis X. In case of focusing in the direction of from infinite to neardistance, the rear group G₃ is shifted toward the object (forward) andsimilarly the corrective group G₂ is shifted toward the imaging side(rearward) to narrow the air space D₂ between the rear group G₃ and thecorrective group G₂. On the contrary, in case of focusing in thedirection of from near distance to infinite, the rear group G₃ isshifted rearward and concurrently the corrective group G₂ is shiftedforward to widen the air space D₂ between the rear group G₃ and thecorrective group G₂.

At the time of focusing, the fixed group G₁ of the front group G₁,2 isheld stationary. The fixation of the larger and heavier group G₁contributes to avoid deteriorations in maneuverability of the lenssystem in focusing. Various aberrations which occur in infinite to neardistance focusing can be effectively corrected by shifting only thecorrective group G₂ of the front group G₁,2 in this manner. The shiftsof the corrective group G₂ and the rear group G₃ may be of either linearor non-linear mode, of which the non-linear mode is preferred from thestandpoint of correction of aberrations.

The fixed subgroup G₁ of the front group G₁,2 is arranged to terminatewith a lens element of a negative refractive power having a concavesurface on the side of the image-forming plane and the corrective groupG₂ of the front group G₁,2 is arranged to start with a lens element of apositive refractive power having a convex surface on the side of theobject, the air space D₁ which is formed between the fixed group G₁ andthe corrective group G₂ functioning as an air lens which can be usedadvantageously for correction of aberrations.

More specifically, the above-described lens system is preferred to bearranged to satisfy the conditions of

    -0.18≦φ.sub.2 /φ.sub.1,2 ≦0.45       (1)

    0.5≦R.sub.2 /R.sub.1 ≦1.2                    (2)

where φ₁,2 is the overall refractive power of the composite front groupG₁,2 of the lens system, namely, of the fixed group G₁ plus thecorrective group G₂, φ₂ is the refractive power of the corrective groupG₂, and R₁ and R₂ are the radii of curvature of the final surface of thefixed group G₁ of the front group G₁,2 and the initial surface of thecorrective group G₂ of the lens system.

The condition (1) defines the distribution of power to the fixed groupG₁ and the corrective group G₂ which constitute the front group G₁,2. Ifthe value is smaller than the lower limit of the condition (1), itbecomes difficult to correct variations in lateral chromatic aberrationresulting from focusing to a different object distance. On the otherhand, if greater than the upper limit of the condition (1), difficultiesare encountered not only in correcting lateral chromatic aberrationwhich varies with the field angle at a given object distance, but alsoin correcting coma which varies to a greater degree with variations inobject distance in focusing.

The condition (2) defines the shape of the air gap between the fixedgroup G₁ and the corrective group G₂. If the value is smaller than thelower limit of the condition (2), there arises a problem that variationin coma becomes greater when focusing to a different object distance,coupled with a difficulty of machining and polishing the concavemeniscus lens elements in the front group to shape. If greater than theupper limit of the condition (2), coma flare will become too large tosecure satisfactory performance quality.

The retrofocus type wide angle lens of the invention is illustrated moreparticularly by the following description of preferred embodiments, inwhich various notations are used to stand for the meanings as follows:

m: The surface number counted sequentially from the object side;

ri: The radius of curvature of a lens component of number i counted fromthe object side;

di: The width of air space or thickness of a lens component of number icounted from the object side;

n_(ei) : The index of refraction, with respect to line e, of a lenscomponent of number i counted from the object side;

ν_(ei) : The abbe number of a lens component of number i counted fromthe object side;

f: Focal length;

Bf: Back focal length;

F: Effective f number; and

Object distance: The distance from the apex of the front lens group ofthe lens system to an object.

EMBODIMENT 1

The lens construction of the Embodiment 1 is shown in FIG. 2.

As clear therefrom, the fixed group G₁ in the front group G₁,2 of thelens system is constituted, when viewed from the object side, by anegative meniscus lens L₁ having a convex surface disposed on the objectside, a negative meniscus lens L₂ having a convex surface disposed onthe object side, a biconvex lens L₃, and a negative meniscus lens L₄having a convex surface disposed on the object side. The correctivegroup G₂ is constituted by a biconvex lens L₅ having a strong convexsurface disposed on the object side, a negative meniscus lens L₆ havinga convex surface disposed on the object side, and a cemented lens L₇₊₈composed of a biconvex lens L₇ and a biconcave lens L₈. The rear groupG₃ is constituted by a cemented lens L₉₊₁₀ composed of a biconcave lensL₉ and a biconvex lens L₁₀, a cemented lens L₁₁₊₁₂ composed of anegative meniscus lens L₁₁ having a convex surface disposed on theobject and a biconvex lens L₁₂, a cemented lens L₁₃₊₁₄ composed of apositive meniscus lens L₁₃ having a concave surface disposed on theobject side and a negative meniscus lens L₁₄ having a concave surfacedisposed on the object side, a cemented lens L₁₅₊₁₆ composed of abiconcave lens L₁₅ and a biconvex lens L₁₆, a positive meniscus lens L₁₇having a concave surface disposed on the object side, and a biconvexlens L₁₈. The lens elements L₁₉ and L₂₀ are employed as a prism P, andan aperture stop S is interposed between the lens elements L₁₀ and L₁₁.

The numeric particulars of this lens construction are given below.

    ______________________________________                                        m       r         d          n.sub.e   ν .sub.d                            ______________________________________                                         1      113.016   3.5        1.77620   49.6                                    2      52.644    12.4                                                         3      133.747   3.0        1.82017   46.6                                    4      52.779    11.4                                                         5      265.171   12.2       1.48914   70.4                                    6      -103.514  0.3                                                          7      93.894    2.2        1.85026   41.7                                    8      48.944    Variable                                                     9      38.601    14.7       1.57561   44.7                                   10      -147.656  0.2                                                         11      46.154    2.0        1.88814   40.8                                   12      18.034    6.8                                                         13      85.695    6.7        1.85501   23.9                                   14      -28.235   2.0        1.82548   45.3                                   15      27.853    Variable                                                    16      -88.664   2.0        1.73233   54.7                                   17      17.089    6.3        1.72310   29.5                                   18      -85.172   12.5                                                        19      538.373   3.5        1.88814   40.8                                   20      22.730    8.7        1.51976   52.4                                   21      -39.609   0.3                                                         22      -388.202  6.8        1.49845   81.6                                   23      -19.124   2.5        1.85649   32.3                                   24      -43.720   0.3                                                         25      -2222.300 2.5        1.85649   32.3                                   26      91.358    5.8        1.49845   81.6                                   27      -64.337   2.2                                                         28      -194.823  7.1        1.48915   70.2                                   29      -29.043   0.3                                                         30      70.108    7.1        1.71615   53.9                                   31      -257.887  2.0                                                         32      ∞   33.0       1.58566   46.4                                   33      ∞   13.2       1.51872   64.0                                   34      ∞   7.2                                                         ______________________________________                                        Focal length f = 5.0 mm                                                       Field angle =  95.5°                                                   F = 1.8                                                                       ______________________________________                                        Object distance  ∞                                                                              270 mm                                                d.sub.8           2.16  6.29                                                  d.sub.15         43.56  38.79                                                 Bf               38.65  39.29                                                 ______________________________________                                        φ.sub.2 /φ.sub.1.2 = 0.31                                             R.sub.2 /R.sub.1 = 0.79                                                       ______________________________________                                    

The aberration curves of this lens construction are shown in FIGS. 3 and4 where the object distance is ∞ and 270 mm, respectively. FIG. 5 shows,for the purpose of comparison, similar aberration curves at the objectdistance of 270 mm, as observed when the whole lens system of the sameconstruction as in the Embodiment 1 is shifted forward for focusing.

EMBODIMENT 2

The lens construction of the Embodiment 2 is shown in FIG. 6.

As clear therefrom, the fixed group G₁ in the front group G₁,2 of thelens system is constituted, when viewed from the object side, by anegative meniscus lens L₁ having a convex surface disposed on the objectside, a biconvex lens L₂, a negative meniscus lens L₃ having a convexsurface disposed on the object side, and a negative meniscus lens L₄having a convex surface disposed on the object side. The correctivegroup G₂ is constituted by a biconvex lens L₅ having a strong convexsurface disposed on the object side, a negative meniscus lens L₆ havinga convex surface disposed on the object side, and a cemented lens L₇₊₈composed of a biconvex lens L₇ and a biconcave lens L₈. The rear groupG₃ is constituted by a cemented lens L₉₊₁₀ composed of a biconcave lensL₉ and a biconvex lens L₁₀, a negative meniscus lens L₁₁ having aconcave surface disposed on the object, a biconvex lens L₁₂ , a positivemeniscus lens L₁₃ having a concave surface disposed on the object side,a cemented lens L₁₄₊₁₅ composed of a negative meniscus lens L₁₄ having aconvex surface disposed on the object side and a biconvex lens L₁₅, acemented lens L₁₆₊₁₇ composed of a negative meniscus lens L₁₆ having aconvex surface disposed on the object side and a biconvex lens L₁₇, anda biconvex lens L₁₈. The lens elements L₁₉ and L₂₀ are employed as aprism P, and an aperture stop S is interposed between the lens elementsL₁₀ and L₁₁.

The numeric particulars of this lens construction are given below.

    ______________________________________                                        m       r         d          n.sub.e   ν .sub.d                            ______________________________________                                         1      139.909   3.5        1.77620   49.6                                    2      67.342    14.5                                                         3      195.911   12.2       1.51825   64.1                                    4      -315.719  0.2                                                          5      92.159    2.4        1.88814   40.8                                    6      43.992    8.7                                                          7      101.956   2.3        1.82548   45.3                                    8      61.819    Variable                                                     9      45.618    12.0       1.63509   35.8                                   10      -314.075  0.3                                                         11      83.874    2.1        1.88814   40.8                                   12      29.637    6.5                                                         13      104.710   10.5       1.81264   25.4                                   14      -36.407   2.1        1.88814   40.8                                   15      63.906    Variable                                                    16      -1305.900 2.0        1.72309   49.1                                   17      26.669    11.0       1.72310   29.5                                   18      -244.808  15.6                                                        19      -34.922   5.7        1.88814   40.8                                   20      -934.482  0.8                                                         21      461.757   9.5        1.49845   81.6                                   22      -46.269   0.2                                                         23      -820.558  8.8        1.49845   81.6                                   24      -67.528   0.2                                                         25      7538.900  2.2        1.85649   32.3                                   26      88.227    11.0       1.48915   70.2                                   27      -88.224   0.2                                                         28      114.166   2.3        1.85649   32.3                                   29      56.804    11.8       1.48915   70.2                                   30      -145.041  0.2                                                         31      62.986    10.7       1.71615   53.9                                   32      -518.147  2.0                                                         33      ∞   50.5       1.69979   55.4                                   34      ∞   18.5       1.51872   64.0                                   35      ∞   16.0                                                        ______________________________________                                        Focal length f = 9.7 mm                                                       Field angle = 80.2°                                                    F = 1.2                                                                       ______________________________________                                        Object distance  ∞                                                                              450 mm                                                d.sub.8           3.26   7.16                                                 d.sub.15         66.80  62.28                                                 Bf               59.91  60.53                                                 ______________________________________                                        φ.sub.2 /φ.sub.1.2 = 0.20                                             R.sub.2 /R.sub.1 = 0.74                                                       ______________________________________                                    

The aberration curves of this lens construction are shown in FIGS. 7 and8 where the object distance is ∞ and 450 mm, respectively. For thepurpose of comparison, FIG. 9 shows similar aberration curves at theobject distance of 450 mm, as observed when the whole lens system of thesame construction as in the Embodiment 2 is shifted forward forfocusing.

EMBODIMENTS 3

The lens construction of the Embodiment 3 is shown in FIG. 10.

As clear therefrom, the fixed group G₁ in the front group G₁,2 of thelens system is constituted, when viewed from the object side, by anegative meniscus lens L₁ having a convex surface disposed on the objectside, a biconvex lens L₂, and a negative meniscus lens L₃ having aconvex surface disposed on the object side. The corrective group G₂ isconstituted by a biconvex lens L₄, a negative meniscus lens L₅ having aconvex surface disposed on the object side, and a cemented lens L₆₊₇composed of a biconvex lens L₆ and a biconcave lens L₇. The rear groupG₃ is constituted by a cemented lens L₈₊₉ composed of a biconcave lensL₈ and a biconvex lens L₉, a cemented lens L₁₀₊₁₁ composed of abiconcave lens L₁₀ and a biconvex lens L₁₁, a positive meniscus lens L₁₂having a concave surface disposed on the object, a biconvex lens L₁₃, anegative meniscus lens L₁₄ having a convex surface disposed on theobject side, a biconvex lens L₁₅, and a biconvex lens L₁₆. The lenselements L₁₇ and L₁₈ are employed as a prism P, and an aperture stop Sis interposed between the lens elements L₉ and L₁₀.

The numeric particulars of this lens construction are given below.

    ______________________________________                                        m       r         d          n.sub.e   ν .sub.d                            ______________________________________                                         1      75.464    3.5        1.77620   49.6                                    2      37.173    13.5                                                         3      653.666   9.0        1.48915   70.2                                    4      -83.737   0.2                                                          5      262.929   2.4        1.82548   45.3                                    6      28.146    Variable                                                     7      28.216    13.5       1.60718   38.0                                    8      -104.871  0.3                                                          9      41.680    2.1        1.88814   40.8                                   10      21.541    6.5                                                         11      43.716    7.5        1.81264   25.4                                   12      -38.832   2.1        1.88814   40.8                                   13      27.365    Variable                                                    14      -66.789   2.0        1.72309   49.1                                   15      28.470    7.0        1.72310   29.5                                   16      -88.466   13.2                                                        17      -85.462   2.5        1.81264   25.4                                   18      74.530    0.8        1.53430   48.9                                   19      -53.143   3.7                                                         20      -129.855  4.0        1.48915   70.2                                   21      -98.779   0.2                                                         22      -75.538   6.5        1.49845   81.6                                   23      -83.118   0.2                                                         24      71.723    2.0        1.85649   32.3                                   25      41.188    2.3                                                         26      53.646    6.0        1.49845   81.6                                   27      -124.513  0.2                                                         28      35.588    7.0        1.49845   81.6                                   29      -2530.300 2.0                                                         30      ∞   33.0       1.58566   46.4                                   31      ∞   13.2       1.51872   64.0                                   32      ∞   8.3                                                         ______________________________________                                        Focal length f = 8.0 mm                                                       Field angle = 69.0°                                                    F = 1.4                                                                       ______________________________________                                        Object distance  ∞                                                                              240 mm                                                d.sub.6           1.04  1.36                                                  d.sub.13         43.17  42.57                                                 Bf               39.78  40.06                                                 ______________________________________                                        φ.sub.2 /φ.sub.1.2 = -0.124                                           R.sub.2 /R.sub.1 = 1.002                                                      ______________________________________                                    

The aberration curves of this lens construction are shown in FIGS. 11and 12 where the object distance is ∞ and 240 mm, respectively. For thepurpose of comparison, FIG. 13 shows similar aberration curves at theobject distance of 240 mm, as observed when the whole lens system of thesame construction as in the Embodiment 3 is shifted forward forfocusing.

EMBODIMENT 4

The lens construction of the Embodiment 3 is shown in FIG. 14.

As clear therefrom, the fixed group G₁ in the front group G₁,2 of thelens system is constituted, when viewed from the object side, by anegative meniscus lens L₁ having a convex surface disposed on the objectside, a biconvex lens L₂, and a negative meniscus lens L₃ having aconvex surface disposed on the object side. The corrective group G₂ isconstituted by a biconvex lens L₄ having a strong convex surface on theobject side, a negative meniscus lens L₅ having a convex surfacedisposed on the object side, and a cemented lens L₆₊₇ composed of abiconvex lens L₆ and a biconcave lens L₇. The rear group G₃ isconstituted by a cemented lens L₈₊₉ composed of a biconcave lens L₈ anda positive meniscus lens L₉ having a convex surface disposed on theobject side, a cemented lens L₁₀₊₁₁ composed of a biconcave lens L₁₀ anda biconvex lens L₁₁, a positive meniscus lens L₁₂ having a concavesurface disposed on the object, a biconvex lens L₁₃, a negative meniscuslens L₁₄ having a convex surface disposed on the object side, a biconvexlens L₁₅, and a biconvex lens L₁₆. The lens elements L₁₇ and L₁₈ areemployed as a prism P, and an aperture stop S is interposed between thelens elements L₉ and L₁₀.

The numeric particulars of this lens construction are given below.

    ______________________________________                                        m       r         d          n.sub.e   ν .sub.d                            ______________________________________                                         1      121.106   3.5        1.77620   49.6                                    2      51.885    14.5                                                         3      314.112   10.5       1.48915   70.2                                    4      -123.225  0.2                                                          5      119.817   2.4        1.82548   45.3                                    6      37.299    Variable                                                     7      37.044    16.5       1.59667   35.3                                    8      -198.975  0.3                                                          9      66.664    2.1        1.88814   40.8                                   10      25.949    6.5                                                         11      65.131    10.5       1.81264   25.4                                   12      -41.500   2.1        1.88814   40.8                                   13      56.384    Variable                                                    14      -174.913  2.0        1.72309   49.1                                   15      37.755    11.0       1.72310   29.5                                   16      271.341   12.2                                                        17      -133.844  3.1        1.81264   25.4                                   18      103.876   25.0       1.53430   48.9                                   19      -66.920   3.5                                                         20      -583.432  6.7        1.48915   70.2                                   21      -175.412  0.3                                                         22      89.254    12.2       1.49845   81.6                                   23      -129.836  0.3                                                         24      82.896    2.9        1.83931   37.2                                   25      45.162    2.4                                                         26      48.245    13.5       1.49845   81.6                                   27      -317.163  0.2                                                         28      73.299    9.0        1.49845   81.6                                   29      -4153.700 2.0                                                         30      ∞   50.5       1.69979   55.4                                   31      ∞   18.5       1.51872   64.0                                   32      ∞   16.1                                                        ______________________________________                                        Focal length f = 13.3 mm                                                      Field angle = 63.2°                                                    F = 1.2                                                                       ______________________________________                                        Object distance                                                               28                      450 mm                                                d.sub.6           4.00   5.25                                                 d.sub.13         47.98  46.21                                                 Bf               59.98  60.51                                                 ______________________________________                                        φ.sub.2 /φ.sub.1.2 = 0.0041                                           R.sub.2 /R.sub.1 = 0.99                                                       ______________________________________                                    

The aberration curves of this lens construction are shown in FIGS. 15and 16 where the object distance is ∞ and 450 mm, respectively. For thepurpose of comparison, FIG. 17 shows similar aberration curves at theobject distance of 240 mm, as observed when the whole lens system of thesame construction as in the Embodiment 4 is shifted forward forfocusing.

As clear from the aberration diagrams of Embodiments 1 to 4, the lenssystem of the present invention is well corrected for variousaberrations including field curvature.

The prism P which is employed for color resolution is not essential toachieve the objectives and effects of the present invention. The stop Sis preferred to be located between the lens elements of the rear groupG₃ from the standpoint of correction of aberrations.

What is claimed is:
 1. A retrofocus type wide angle lens system composedof a negative front group G₁,2 and a positive rear group G₃,characterized in that:said front group G₁,2 is constituted by a fixedgroup G₁ starting with a negative meniscus lens having a convex surfaceon the object side and terminating with a negative lens element having aconcave surface on the side of an image-forming plane, and a correctivegroup G₂ starting with a positive lens element having a convex surfaceon the object side, said fixed group G₁ being fixed relative to saidimage-forming plane while said corrective group G₂ and said rear groupG₃ being movable in the direction of the optical axis to vary the widthof the air space between the respective lens groups for focusingpurposes.
 2. A retrofocus type wide angle lens system as defined inclaim 1, wherein said lens system is arranged to satisfy the conditionsof

    -0.18≦φ.sub.2 /φ.sub.1,2 ≦0.45       (1)

    0.5≦R.sub.2 /R.sub.1 ≦1.2                    (2)

where φ₁,2 is the overall refractive power of the composite front groupG₁,2 of said lens system including said fixed group G₁ and saidcorrective group G₂, φ₂ is the refractive power of said corrective groupG₂, and R₁ and R₂ are the radii of curvature of the final surface ofsaid fixed group G₁ of said front group G₁,2 and the initial surface ofsaid corrective group G₂ of said lens system.
 3. A retrofocus type wideangle lens system a defined in claim 1, wherein:said fixed group G₁ insaid front group G₁,2 of said lens system is constituted, when viewedfrom the object side, by a negative meniscus lens L₁ having a convexsurface disposed on the object side, a negative meniscus lens L₂ havinga convex surface disposed on the object side, a biconvex lens L₃, and anegative meniscus lens L₄ having a convex surface disposed on the objectside; said corrective group G₂ is constituted by a biconvex lens L₅having a strong convex surface disposed on the object side, a negativemeniscus lens L₆ having a convex surface disposed on the object side,and a cemented lens L₇₊₈ composed of a biconvex lens L₇ and a biconcavelens L₈ and said rear group G₃ is constituted by a cemented lens L₉₊₁₀composed of a biconcave lens L₉ and a biconvex lens L₁₀, a cemented lensL₁₁₊₁₂ composed of a negative meniscus lens L₁₁ having a convex surfacedisposed on the object and a biconvex lens L₁₂, a cemented lens L₁₃₊₁₄composed of a positive meniscus lens L₁₃ having a concave surfacedisposed on the object side and a negative meniscus lens L₁₄ having aconcave surface disposed on the object side, a cemented lens L₁₅₊₁₆composed of a biconcave lens L₁₅ and a biconvex lens L₁₆, a positivemeniscus lens L₁₇ having a concave surface disposed on the object side,and a biconvex lens L₁₈.
 4. A retrofocus type wide angle lens system asdefined in claim 1, wherein:said fixed group G₁ in said front group G₁,2of said lens system is constituted, when viewed from the object side, bya negative meniscus lens L₁ having a convex surface disposed on theobject side, a biconvex lens L₂, a negative meniscus lens L₃ having aconvex surface disposed on the object side, and a negative meniscus lensL₄ having a convex surface disposed on the object side; said correctivegroup G₂ is constituted by a biconvex lens L₅ having a strong convexsurface disposed on the object side, a negative meniscus lens L₆ havinga convex surface disposed on the object side, and a cemented lens L₇₊₈composed of a biconvex lens L₇ and a biconcave lens L₈ ; and said reargroup G₃ is constituted by a cemented lens L₉₊₁₀ composed of a biconcavelens L₉ and a biconvex lens L₁₀, a negative meniscus lens L₁₁ having aconcave surface disposed on the object, a biconvex lens L₁₂, a positivemeniscus lens L₁₃ having a concave surface disposed on the object side,a cemented lens L₁₄₊₁₅ composed of a negative meniscus lens L₁₄ having aconvex surface disposed on the object side and a biconvex lens L₁₅, acemented lens L₁₆₊₁₇ composed of a negative meniscus lens L₁₆ having aconvex surface disposed on the object side and a biconvex lens L₁₇, anda biconvex lens L₁₈.
 5. A retrofocus type wide angle lens system asdefined in claim 1, wherein:said fixed group G₁ in said front group G₁,2of said lens system is constituted, when viewed from the object side, bya negative meniscus lens L₁ having a convex surface disposed on theobject side, a biconvex lens L₂, and a negative meniscus lens L₃ havinga convex surface disposed on the object side; said corrective group G₂is constituted by a biconvex lens L₄, a negative meniscus lens L₅ havinga convex surface disposed on the object side, and a cemented lens L₆₊₇composed of a biconvex lens L₆ and a biconcave lens L₇ ; and said reargroup G₃ is constituted by a cemented lens L₈₊₉ composed of a biconcavelens L₈ and a biconvex lens L₉, a cemented lens L₁₀₊₁₁ composed of abiconcave lens L₁₀ and a biconvex lens L₁₁, a positive meniscus lens L₁₂having a concave surface disposed on the object, a biconvex lens L₁₃, anegative meniscus lens L₁₄ having a convex surface disposed on theobject side, a biconvex lens L₁₅, and a biconvex lens L₁₆.
 6. Aretrofocus type wide angle lens system as defined in claim 1,wherein:said fixed group G₁ in said front group G₁,2 of said lens systemis constituted, when viewed from the object side, by a negative meniscuslens L₁ having a convex surface disposed on the object side, a biconvexlens L₂, and a negative meniscus lens L₃ having a convex surfacedisposed on the object side; said corrective group G₂ is constituted bya biconvex lens L₄ having a strong convex surface on the object side, anegative meniscus lens L₅ having a convex surface composed of a biconvexlens L₆ and a biconcave lens L₇ ; and said rear group G₃ is constitutedby a cemented lens L₈₊₉ composed of a biconcave lens L₈ and a positivemeniscus lens L₉ having a convex surface disposed on the object side, acemented lens L₁₀₊₁₁ composed of a biconcave lens L₁₀ and a biconvexlens L₁₁, a positive meniscus lens L₁₂ having a concave surface disposedon the object, a biconvex lens L₁₃, a negative meniscus lens L₁₄ havinga convex surface disposed on the object side, a biconvex lens L₁₅, and abiconvex lens L₁₆.
 7. A retrofocus type wide angle lens system asdefined in claim 1, wherein an aperture is interposed between lenselements of said rear group G₃.